Trapeziometacarpal joint implant and associated methods

ABSTRACT

A trapeziometacarpal joint implant includes a body defining a median plane, a metacarpal joint surface, and a trapezium joint surface. A first central region of the metacarpal joint surface is situated on an opposite side of the median plane from a second central region of the trapezium joint surface. The first and second central regions correspond to profiles of a first axial segment and a second axial segment, respectively. The first and second axial segments are one of a cylinder, a cone and a torus and are centered on a first axis and a second axis, respectively, where the first and second axes, as projected on the median plane, are substantially perpendicular to each other.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of French Patent ApplicationNo. FR0954190, filed on Jun. 19, 2009, and entitled “IMPLANTD′ARTICULATION TRAPÉZO-MÉTACARPIENNE,” which is incorporated herein byreference in its entirety for all purposes.

BACKGROUND

Trapeziometacarpal prostheses and implants are designed to restore thestrength and mobility of the anatomical joint of the thumb, between itsmetacarpal and the trapezium, when the joint is damaged by adegenerative or inflammatory pathological process. Manytrapeziometacarpal prostheses are stem prostheses in which themetacarpal prosthetic component is anchored at one end in the metacarpalby an elongate stem while the other end of the prosthetic component ismounted pivotably either in a prosthetic component attached to thetrapezium or in a cavity hollowed out in the trapezium. The implantationof these total or partial prostheses requires considerable cutting ofthe bone substance of the trapeziometacarpal joint and restores aball-and-socket type of mobility which, from the point of view ofkinematics, does not correspond to the mobility of the anatomical joint.Moreover, in the event of poor implantation, there are real risks ofwear, luxation, or even prosthesis fracture. Some trapeziometacarpalimplants without stems have also been proposed, those implants beingimplanted into a cavity or cavities formed in the trapezium and/ormetacarpal. Often times, because of the compressive stresses applied tothe implant body by the surrounding ligaments of the joint, the implantbody gradually engages deeper in the bones, reducing the space betweenthe bones and, consequently, their relative mobility.

SUMMARY

Some embodiments relate to a trapeziometacarpal implant that ispermanently or semi-permanently implantable and provides mobility moresimilar to natural, anatomical mobility of the trapeziometacarpal joint.The trapeziometacarpal joint implant includes a body defining ametacarpal joint surface and a trapezium joint surface that aresituated, at least in their central region, on either side of a medianplane of the body. Each of the metacarpal and the trapezium jointsurfaces correspond to a profile of an axial segment of a cylinder, acone, or a torus, the axial segments of the cylinder, cone and/or torusbeing centered on respective axes which, as projected on the medianplane, are substantially perpendicular to each other.

Some embodiments relate to a surgical method for fitting atrapeziometacarpal joint implant in which an articular area between themetacarpal and trapezium of a patient is accessed, the cortical bonesurface of the trapezium is prepared so as to substantially match thetrapezium joint surface defined by a body of an implant and, if need be,the cortical bone surface of the metacarpal is prepared so as tosubstantially match a metacarpal joint surface defined by the body ofthe implant, and the body of the implant is interposed between thetrapezium and the metacarpal in such a way that an axis associated withthe trapezium joint surface extends in either the antero-posterior orfrontal plane of the patient, while an axis associated with themetacarpal joint surface extends in the other of the antero-posterior orfront planes.

Some embodiments relate to interposing between the trapezium and themetacarpal a relatively thin body (e.g., as opposed to a body that isvoluminous and spherical or ellipsoid) with opposite joint surfaceswhich are concave at least in one direction and against which thecortical bone surfaces of the trapezium and of the metacarpal come tobear so as to roll and slide thereon. In order to reproduce theanatomical behavior of the saddle-type trapeziometacarpal joint, the twojoint surfaces correspond to profiles of axial segments of a cylinder,cone and/or torus, of which the central axes or, more generally, takingaccount that the central axis of an axial segment of a torus is curved,the projections of these central axes on the median plane of theimplant, are substantially perpendicular to each other.

In some embodiments, the body of the implant simulates the anatomicalsaddle-type trapeziometacarpal joint with the two joint surfaces. Fromthe point of view of kinematics, the two, orthogonal joint surfaces ofthe implant body permit pivoting mobility about the two aforementionedcentral axes, in the manner of a cardan joint, or universal joint, towhich the trapeziometacarpal joint is similar from a mechanical point ofview. Moreover, the curvature of the two joint surfaces stabilizes andaligns the body of the implant between the trapezium bone and themetacarpal bone, thus avoiding the need to provide means of fixation tothe bone(s). Moreover, by maximizing an area of contact between theimplant and the bones of the trapezium and metacarpal in the area of thetwo joint surfaces, the implant body is adapted to reduce the likelihoodof being forced into the bones, thereby retaining mobility of the joint.

In some embodiments, the body of the implant is minimized in thickness,where a range of thicknesses are optionally offered to a surgeon duringimplantation depending on a degree of wear of the joint that is beingtreated. Generally, the natural contours of the trapezium and metacarpalcortical bone surfaces are well-matched for cooperating with the jointsurfaces of the implant. In cases of substantial wear, however,minimizing thickness of the implant helps minimize an amount of bonecutting required to match the mutually facing cortical bone surfaces ofthe trapezium and of the metacarpal to the joint surfaces of the body ofthe implant. In turn, for some trapeziometacarpal joints that are notgreatly damaged, resection of the metacarpal can be minimal or evenunnecessary. Typically, the size of the implant is selected such thatthe implant body is stabilized by the stress provided by the capsularand ligament envelope around the joint between the trapezium and themetacarpal, without the implant body being overstressed. For example, insome embodiments, the implant body retains a certain degree of freedomof movement to help the implant adjust its position, or adapt, as afunction of the stresses of the articular envelope associated with thevarious possible movements of the trapeziometacarpal joint.

Various embodiments relate to a trapeziometacarpal joint implant havinga trapezium joint surface and a metacarpal joint surface opposite thetrapezium joint surface. In some embodiments, one or both of themetacarpal and trapezium joint surfaces correspond to a profile of anaxial segment of a cylinder, a cone, or a torus having a cross sectionthat is curved, curved inward along its entire periphery, elliptic,and/or circular. In some embodiments, the cross section associated withthe metacarpal and/or trapezium joint surface has, in the central regionof the joint surface, a smaller curvature than a remainder of the crosssection and, if desired, in the peripheral region of the joint surface,a greater curvature than a remainder of the cross section. Themetacarpal and trapezium joint surfaces are separated from each other,in a direction perpendicular to a median plane of the implant, by amaximum thickness of the body of less than 5 mm, according to someembodiments. The trapezium joint surface optionally has a dimension,along its associated axis, greater than the dimension of the metacarpaljoint surface along its associated axis. In some embodiments, the bodyis without any means of fixation to bone and the implant body is formedas a single, unitary component.

While multiple embodiments are disclosed, still other embodiments of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the invention. Accordingly, the drawings anddetailed description are to be regarded as illustrative in nature andnot restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view showing an implant interposed between atrapezium and metacarpal of a human hand, according to some embodiments.

FIG. 2 is a perspective view of the implant of FIG. 1, according to someembodiments.

FIG. 3 is an elevation in the direction of the arrow III in FIG. 2,according to some embodiments.

FIGS. 4 and 5 are sections along the lines IV-IV and V-V, respectively,from FIG. 3, also showing some of the bones of the hand after theimplant of FIG. 1 has been positioned, according to some embodiments.

FIG. 6 is a diagram illustrating a design geometry of joint surfaces ofthe implant of FIG. 1, according to some embodiments.

FIGS. 7 to 9 are diagrams illustrating other design geometries of jointsurfaces of other implants, according to some embodiments.

While the invention is amenable to various modifications and alternativeforms, specific embodiments have been shown by way of example in thedrawings. The intention, however, is not to limit the invention to theparticular embodiments depicted. On the contrary, the invention isintended to cover all modifications, permutations, equivalents, andalternatives falling within the scope of the invention as defined by theappended claims.

DETAILED DESCRIPTION

As used herein, the terms “frontal,” “antero-posterior,” and similarterms are to be understood in their anatomical sense in relation to apatient whose hand is being operated upon. With that in mind, in FIGS. 1to 5, a trapeziometacarpal implant 1 is shown which is designed to beimplanted by interposition in a trapeziometacarpal joint between atrapezium T and a first metacarpal M of a human hand, as is shown by wayof example in FIG. 1.

The implant 1 comprises a generally disk-shaped body 2 centered on ageometric axis Z-Z, also described as a transverse axis. The body 2delimits a first main surface 4 and a second main surface 6, alsoreferred to as metacarpal and trapezium joint surfaces, respectively, orprimary bone contact surfaces, for example. In some embodiments, thefirst and second main surfaces 4 and 6 are positioned opposite eachother on the axis Z-Z and separated from each other by the thickness ofthe body 2 along the axis Z-Z. The body 2 also includes a peripheralsurface 8 that connects the first and second main surfaces 4 and 6. Insome embodiments, the generatrices of the peripheral surface 8 areparallel to the axis Z-Z.

As shown, the transverse profile of the body 2 is generally rectangular.In particular, in a cross section that is transverse to the axis Z-Z,the body 2 has a rectangular outer contour with rounded corners,although a variety of shapes are contemplated, including square,circular, elliptical, and others, for example.

As shown, the implant 1 is formed as a single, unitary piece, consistingof the body 2 without extraneous components, such as a bone fixationcomponent (e.g., a screw) for anchoring the body 2 in the metacarpal Mor in the trapezium T. For example, in some embodiments, the body 2 issecured in position via interposition between metacarpal M and thetrapezium T such that the body retains some freedom of movement to adaptits position to the stresses applied to it as a function of movements ofthe trapeziometacarpal joint.

In some embodiments, the body 2 is formed as a single piece of graphitecovered with a layer of pyrolytic carbon, which provides goodbiocompatibility, high mechanical strength, and resists wearing of themetacarpal and the trapezium. Although graphite and pyrolytic carbonhave been referenced, other materials are contemplated, including metalssuch as chromium cobalt alloys and plastics, such as polyethylene, PEEK,silicone, as well as ceramics and others.

As shown in FIG. 4, the first main surface 4 is concave and correspondsgeometrically to a profile of an axial segment of a cylinder portionwhich is centered on a first axis X₄ and defines a first cross sectionS₄ that is substantially elliptic. In some embodiments, the first crosssection S₄ is constant along the first axis X₄ and corresponds to thedirectrix of the first main surface 4. As shown, the first axis X₄extends perpendicular to the axis Z-Z, the axes X₄ and Z-Z intersectingwith one another (for reference, the first axis X₄ extends in and out ofthe page as depicted in FIG. 4).

As shown in FIG. 5, the second main surface 6 is also concave andcorresponds to a cylinder portion whose second cross section S₆ issubstantially elliptic and which is centered on a second axis X₆perpendicular to and intersecting the axis Z-Z (for reference, the axisX₅ extends in and out of the page as depicted in FIG. 5).

Thus, as shown, the axes X₄ and X₆ extend perpendicular to each otherand do not intersect. Base lines 41 (FIG. 2) and 61 (FIG. 3) belongrespectively to the first and second main surfaces 4 and 6. Each of thebase lines 41 and 61 extend parallel to the axes X₄ and X₆ and areperpendicular to each other. The base lines 41 and 61 are separated, onthe axis Z-Z, by a central part 21 of the body 2. In different terms, ina median plane π of the body 2, which is perpendicular to the axis Z-Zand on either side of which the first and second main surfaces 4 and 6are situated, the respective projections of the axes X₄ and X₆ areperpendicular and secant.

In some embodiments, in cross section through the axis Z-Z, the body 2has a variable thickness, the thickness being at its minimum in thecentral part 21 of the body and gradually increasing in the directionaway from the central part 21 toward a peripheral part 22 of the body 2,as shown in FIGS. 4 and 5.

The following method of fitting the implant in place between themetacarpal M and the trapezium T is accordance with some embodiments andis provided by way of example. Thus, in some embodiments, a firstoperating step includes a surgeon accessing the articular area betweenthe metacarpal M and the trapezium T via a posterior or antero-lateralapproach. To access the interosseous space between the metacarpal andthe trapezium, the articular capsule and the ligaments surrounding thearticular area are either preserved, by being retracted, or arepartially incised, it being understood that the capsule and itsligaments will then be reconstructed at the end of intervention.

In some embodiments, in a second operating step, the surgeon moves themetacarpal M and the trapezium T apart from each other in order to widenthe interosseous space, in particular by applying a tensile force in thelongitudinal direction of the metacarpal. The surgeon is then betterable to prepare the bone surfaces delimited by the metacarpal M and thetrapezium T, in such a way as to shape these surfaces so that they matchthe first and second main surfaces 4 and 6 of the body 2 of the implant1.

In practice, it will be noted that the mutually facing ends of themetacarpal M and of the trapezium T have respective surface geometriesthat are close to surfaces matching the first and second main surfaces 4and 6. This is because these bone ends, in their anatomical state, havesaddle shapes, which fit orthogonally one into the other. Thus, the cutsmade in the bone on the mutually facing ends of the metacarpal M and thetrapezium T for adapting the surface of the bones M and T to thesurfaces 4 and 6 of the body 2 can be minimal, according to someembodiments. In particular, the cuts are often minimal or evenunnecessary on the metacarpal M, whereas more substantial cuts may haveto be made on the trapezium T. In some embodiments, preparation of theends of the metacarpal M and/or of the trapezium T does not require anydeep cutting into the bones, as cutting remains confined to the corticalbone layer.

In some embodiments, in a third operating step, the tension applied tothe trapeziometacarpal joint in the longitudinal direction of themetacarpal M is maintained. The surgeon positions the implant 1 in theinterosseous space separating the metacarpal M and the trapezium T byinterposing the body 2 between the two bones M and T in such a way thatthe first main surface 4 is directed toward the metacarpal M, with thefirst axis X₄ extending in a frontal plane, while the second mainsurface 6 is directed toward the trapezium T, with the second axis X₆extending in an antero-posterior plane.

In a variant not shown, the implant 1 is fitted in place, between themetacarpal M and the trapezium T, in a configuration tilted by 90degrees about the axis Z-Z. The ends of the metacarpal M and of thetrapezium T are prepared in advance of fitting the implant 1, accordingto some embodiments. Regardless, the body 2 is optionally implanted insuch a way that the first and second main surfaces 4 and 6 adapt betterto the ends of the metacarpal M and trapezium T of the patient beingoperated on, depending on the initial state of these bone ends, and/orto minimize a depth to which the bone ends are cut during preparation.

Once the implant 1 has been positioned, or interposed, between themetacarpal M and the trapezium T, the axial tension applied to thetrapeziometacarpal joint is released, such that the articular capsuleand the ligaments surrounding the trapeziometacarpal joint move themetacarpal M and the trapezium T toward each other. In some embodiments,the body 2 of the implant 1 and/or the metacarpal M and trapezium T areadapted such that the implant 1 is held movably between the metacarpal Mand the trapezium T, under the stress provided by the capsule and theligaments. In other words, the capsule and the ligaments provide astress whose resultant force, or a substantial component thereof, issubstantially aligned with the axis Z-Z. The surgeon then closes thesoft tissues around the trapeziometacarpal joint, if appropriate byreconstruction, or by ligamentoplasty, for example.

The trapeziometacarpal joint thus fitted with the implant 1 exhibits akinematic behavior similar to, or even nearly identical to the naturalanatomical behavior of the trapeziometacarpal joint. For example, theimplant 1 is configured such that the metacarpal M articulates againstthe first main surface 4 by tilting about the first axis X₄ whereas thetrapezium T concurrently articulates against the second main surface 6by tilting about the second axis X₆. By virtue of the implant 1, themetacarpal M and the trapezium T are articulated with respect to eachother in the manner of a cardan joint, or universal joint, about the twoperpendicular axes X₄ and X₆. These cardan joint kinematics efficientlyreproduce the anatomical, saddle-type joint typically present betweenthe metacarpal M and the trapezium T. Moreover, according to someembodiments, the first and second main surfaces 4 and 6 rest against therespective cortical bone layers of the metacarpal M and of the trapeziumT, which helps prevent the body 2 from being forced into, or furtherpenetrating one and/or the other of the metacarpal and trapezium bones Mand T. By reducing the risk of this sinking effect of the implant 1 intobone, the mobility afforded by the implant 1 is longer lasting, andpotentially lifelong in duration.

In some embodiments, in order to promote the rolling and slidingmovements of the metacarpal M and of the trapezium T against therespective central parts 42 of the first and second main surfaces 4 and6, the first and second, elliptic cross sections S₄ and S₆ of the firstand second main surfaces 4 and 6 have, in the central regions 42 and 62,a smaller curvature than a remainder of the cross section. In otherwords, the central regions 42 and 62 of the first and second mainsurfaces 4 and 6 are more flattened relative to the surrounding portionsof the implant 1, such as the peripheral regions 43 and 63, the centralregions 42 and 62 being less curved than the rest of these surfaces. Insome embodiments, the difference in curvature of the first and secondcross sections S₄ and S₆ of the first and second main surfaces 4 and 6between the central regions 42 and 62 and the peripheral regions 43 and63 of the implant is relatively minor, while still having the desiredeffect.

In some embodiments, when in use, the body 2 is stabilized between themetacarpal M and the trapezium T on account of the elliptic curvature ofthe respective first and second cross sections S₄ and S₆ of the firstand second main surfaces 4 and 6. In order to reinforce this stability,the first and second cross sections S₄ and S₆ of the respectiveperipheral regions 43 and 63 of the first and second main surfaces 4 and6 have a greater curvature than the rest of the cross section. Byexaggerating the curvature of the surfaces 4 and 6 in the area of theirperiphery, the surface cooperation of the body 2 with the metacarpal Mand the trapezium T self-stabilizes the implant 1. Furthermore, theperipheral regions 43 and 63 of the surfaces 4 and 6 can thus compensatefor the peripheral wear, associated with arthrosis, of the mutuallyfacing bone ends of the metacarpal M and trapezium T.

As shown, the body 2 is an implant of interposition. Thus, the thicknessof the implant 1 along the axis Z-Z is limited, in the sense that thepresence of the body 2 is adapted to avoid overstressing thetrapeziometacarpal joint. Thus, in some embodiments, the maximumthickness e₂ of the body 2 (e.g., the thickness between the peripheralregions 43 and 63 of the first and second main surfaces 4 and 6), isless than 5 mm, and preferably equal to about 1 mm, although a varietyof dimensions are contemplated. Similarly, in order to adapt optimallyto the interosseous space between the metacarpal M and the trapezium T,the second main surface 6 has a dimension L₆, along its second axis X₆,that is greater than a dimension L₄ of the first main surface 4 alongthe first axis X₄.

The geometry of the first and second main surfaces 4 and 6 areoptionally constructed with different shapes than described above. FIG.6 is a diagram for visualizing a geometry of the first and second mainsurfaces 4 and 6 as shown in FIGS. 1 to 5. As shown in FIG. 6, the firstand second main surfaces 4 and 6 correspond to profiles of axialsegments, or portions, of cylinders defining the first and second axesX₄ and X₆, respectively. As shown, the axial segments have ellipticalbases. In particular, the first and second cross sections S₄ and S₆ ofthe first and second main surfaces 4 and 6 have elliptic shapes centeredon the first axis X₄ and second axis X₆, respectively. Thus, thegeometric definition referenced above for the first and second mainsurfaces 4 and 6 is again shown here in a more schematic form.

FIGS. 7, 8 and 9 are diagrams similar to the one in FIG. 6 showinggeometric variants of the first and second main surfaces 4 and 6 of theimplant 1. In FIG. 7, first and second main surfaces 104 and 106, alsodescribed as metacarpal and trapezium joint surfaces, respectively, arerepresented, each of which correspond to a profile of an axial segmentof a cylinder that is centered on first and second axes X₁₀₄ and X₁₀₆,respectively, and of which the first and second cross sections S₁₀₄ andS₁₀₆, respectively, are circular and centered on the aforementionedaxes. In other words, in some embodiments, the first and second mainsurfaces 4 and 6 and the first and second main surfaces 104 and 106differ only in terms of the geometric shape of their cross section,namely elliptical for the first and second main surfaces 4 and 6 andcircular for the first and second main surfaces 104 and 106.

Similarly, in FIG. 8, first and second main surfaces 204 and 206,respectively, are represented, each of which corresponds to a profile ofan axial segment of a cone that is centered on first and second axesX₂₀₄ and X₂₀₆, respectively, and of which the first and second crosssections S₂₀₄ and S₂₀₆, respectively, are circular. In yet anothervariant not depicted, the aforementioned first and second cross sectionsS₂₀₄ and S₂₀₆ are elliptic, rather than circular.

Likewise, in FIG. 9, first and second main surfaces 304 and 306,respectively, are represented, each of which corresponds to a profile ofan axial segment of a torus that is associated with circular, central,first and second axes X₃₀₄ and X₃₀₆, respectively, and of which thefirst and second cross sections S₃₀₄ and S₃₀₆, respectively, that is tosay the cross section in a plane perpendicular to the applicable axis,is circular and centered on the axes X₃₀₄ and X₃₀₆, respectively. Inother variants not depicted, the first and second cross sections S₃₀₄and S₃₀₆ of the aforementioned torus is made elliptical. Similarly, andonce again in a variant not depicted, the circular cross section or theelliptical cross section of the torus, instead of being constant alongthe first and second axes X₃₀₄ and X₃₀₆, respectively, optionallyincreases or decreases along the first and second axes X₃₀₄ and X₃₀₆.

In still other embodiments, the curved geometry of the first and secondcross sections S₁₀₄, S₁₀₆, S₂₀₄, S₂₀₆, S₃₀₄ and S₃₀₆ are continuouslycircular or elliptic or, by contrast, have substantial variations ofcurvature along their peripheries, for example. In particular, as hasbeen mentioned above for the elliptic geometry of the first and secondcross sections S₄ and S₆, the circular or elliptical geometry of thefirst and second cross sections S₁₀₄, S₁₀₆, S₂₀₄, S₂₀₆, S₃₀₄ and/or S₃₀₆optionally have smaller curvatures in the central regions of thecorresponding first and second main surfaces 104, 106, 204, 206, 304,and 306 and/or have a greater curvature in the peripheral regions of thefirst and second main surfaces 104, 106, 204, 206, 304, and 306 inrelation to a remainder of the cross sections.

FIGS. 6 to 9 thus illustrate various design geometries of the metacarpaland trapezium joint surfaces, or first and second main surfaces of theimplant 1, according to some various embodiments. Depending upon theapplication, the various design geometries provide desirable articularmobility simulating that of the natural trapeziometacarpal joint, oncethe body 2 is implanted, given that the two opposite first and secondmain surfaces 4 and 6, 104 and 106, 204 and 206, 304 and 306 of theimplant 1 are arranged orthogonal to each other. In other words, theopposite first and second main surfaces 4 and 6, 104 and 106, 204 and206, 304 and 306 of the implant 1 are arranged in such a way that theircentral, first and second axes X₄ and X₆, X₁₀₄ and X₁₀₆, X₂₀₄ and X₂₀₆,X₃₀₄ and X₃₀₆ extend substantially perpendicular to one other. Inembodiments with surfaces having corresponding curved axes, such as thecurved, first and second axes X₃₀₄ and X₃₀₆, the feature ofperpendicularity between the particular set of opposing first and secondmain surfaces is exhibited via projection of the axes in the medianplane π of the body 2 of the implant 1, where the respective centralregions of the first and second main surfaces 304 and 306 are arrangedon either side of the plane π (as is also the case with the projectionson the plane π of the rectilinear, first and second axes X₄ and X₆, X₁₀₄and X₁₀₆, X₂₀₄ and X₂₀₆, which are also perpendicular to each other).

Various modifications and additions can be made to the exemplaryembodiments discussed without departing from the scope of the presentinvention. For example, although various embodiments have been describedwith similar, angularly offset shapes for the various first and secondmain surfaces, it is also contemplated that the design geometry of thefirst main surface is the same as, with strictly identical or differentdimensioning, or different than, the design geometry chosen for thesecond main surface of the implant. Additionally, while the embodimentsdescribed above refer to particular features, the scope of thisinvention also includes embodiments having different combinations offeatures and embodiments that do not include all of the above describedfeatures.

The following is claimed:
 1. A trapeziometacarpal joint implant, theimplant comprising a body defining: a median plane, a metacarpal jointsurface and a trapezium joint surface, the metacarpal joint surfacehaving a first central region extending from a center of thetrapeziometacarpal joint implant toward a periphery of thetrapeziometacarpal joint implant and the trapezium joint surface havinga second central region extending from the center of thetrapeziometacarpal joint implant toward the periphery of thetrapeziometacarpal joint implant, the first central region beingsituated on an opposite side of the median plane from the second centralregion, the first and second central regions corresponding to profilesof a first axial segment and a second axial segment, respectively, thefirst and second axial segments each being one of a cylinder, a cone anda torus and being centered on a first axis and a second axis,respectively, the first and second axes, as projected on the medianplane, being perpendicular to each other; wherein the metacarpal jointsurface further defines a first peripheral region adjacent the firstcentral region and the first axial segment has a first cross sectionthat has a smaller radius of curvature in the first central region ofthe metacarpal joint surface than in the first peripheral region of themetacarpal joint surface.
 2. The implant of claim 1, wherein at leastone of the first and second axial segments has a cross section that iscurved inward along an entire periphery thereof.
 3. The implant of claim2, wherein at least one of the first and second axial segments has anelliptic cross section.
 4. The implant of claim 1, wherein at least oneof the first and second axial segments has a circular cross section. 5.The implant of claim 1, wherein the metacarpal joint surface and thetrapezium joint surface are separated from each other, in a directionperpendicular to the median plane by a maximum body thickness of lessthan 5 mm.
 6. The implant of claim 1, wherein the trapezium jointsurface has a length dimension along the first axis that is greater thana length dimension of the metacarpal joint surface along the secondaxis.
 7. The implant of claim 1, wherein the body is free of anycomponents for bone fixation.
 8. The implant of claim 1, wherein thebody is formed as one piece.
 9. The implant of claim 1, wherein themetacarpal joint surface is disposed entirely on a first side of themedian plane, and wherein the trapezium joint surface is disposedentirely on a second side of the median plane opposite the first side.10. The implant of claim 1, wherein the metacarpal joint surface isconfigured to interface with a metacarpal and the trapezium jointsurface is configured to interface with a trapezium.
 11. Atrapeziometacarpal joint implant, the implant comprising a bodydefining: a median plane, a metacarpal joint surface and a trapeziumjoint surface, the metacarpal joint surface having a first centralregion extending from a center of the trapeziometacarpal joint implanttoward a periphery of the trapeziometacarpal joint implant and thetrapezium joint surface having a second central region extending fromthe center of the trapeziometacarpal joint implant toward the peripheryof the trapeziometacarpal joint implant, the first central region beingsituated on an opposite side of the median plane from the second centralregion, the first and second central regions corresponding to profilesof a first axial segment and a second axial segment, respectively, thefirst and second axial segments each being one of a cylinder, a cone anda torus and being centered on a first axis and a second axis,respectively, the first and second axes, as projected on the medianplane, being perpendicular to each other, wherein the metacarpal jointsurface further defines a first peripheral region adjacent the firstcentral region and the first axial segment has a first cross sectionthat has a larger radius of curvature in the first peripheral regionthan in the first central region of the metacarpal joint surface.
 12. Atrapeziometacarpal joint implant, the implant comprising a bodydefining: a median plane, a metacarpal joint surface and a trapeziumjoint surface, the metacarpal joint surface having a first centralregion extending from a center of the trapeziometacarpal joint implanttoward a periphery of the trapeziometacarpal joint implant and thetrapezium joint surface having a second central region extending fromthe center of the trapeziometacarpal joint implant toward the peripheryof the trapeziometacarpal joint implant, the first central region beingsituated on an opposite side of the median plane from the second centralregion, the first and second central regions corresponding to profilesof a first axial segment and a second axial segment, respectively, thefirst and second axial segments each being one of a cylinder, a cone anda torus and being centered on a first axis and a second axis,respectively, the first and second axes, as projected on the medianplane, being perpendicular to each other, wherein the trapezium jointsurface further defines a second peripheral region adjacent the secondcentral region and the second axial segment has a second cross sectionthat has a smaller radius of curvature in the second central region ofthe trapezium joint surface than in the second peripheral region of thetrapezium joint surface.
 13. A trapeziometacarpal joint implant, theimplant comprising a body defining: a median plane, a metacarpal jointsurface and a trapezium joint surface, the metacarpal joint surfacehaving a first central region extending from a center of thetrapeziometacarpal joint implant toward a periphery of thetrapeziometacarpal joint implant and the trapezium joint surface havinga second central region extending from the center of thetrapeziometacarpal joint implant toward the periphery of thetrapeziometacarpal joint implant, the first central region beingsituated on an opposite side of the median plane from the second centralregion, the first and second central regions corresponding to profilesof a first axial segment and a second axial segment, respectively, thefirst and second axial segments each being one of a cylinder, a cone anda torus and being centered on a first axis and a second axis,respectively, the first and second axes, as projected on the medianplane, being perpendicular to each other, wherein the trapezium jointsurface further defines a second peripheral region adjacent the secondcentral region and the second axial segment has a second cross sectionthat has a larger radius of curvature in the second peripheral regionthan in the second central region of the trapezium joint surface.